Renin is an endopeptidase secreted by the juxtaglomerular cells of the kidney, which cleaves its plasma substrate, angiotensinogen, specifically at the 10-11 peptide bond, i.e., between Leu 10 and Leu 11 in the equine substrate, as described by Skeqqs et al, J. Exper. Med. 1957, 106, 439, or between the Leu 10 and Val 11 in the human renin substrate, as elucidated by Tewksbury et al., Circulation 59, 60, Supp. II: 132, Oct. 1979. Renin cleaves angiotensinogen to split off the decapeptide, angiotensin I, which is converted by angiotensin-converting enzyme to the potent presser substance anqiotensin II. Thus, the renin anqiotensin system plays an important role in normal cardiovascular homeostasis and in some forms of hypertension.
Inhibitors of anqiotensin I converting enzyme have proven useful in the modulation of the renin-anqiotensin system and consequently, specific inhibitors of the limiting enzymatic step that ultimately regulates anqiotensin II production, the action of renin on its substrate, have also been sought as effective investigative tools and as therapeutic agents in the treatment of hypertension and congestive heart failure.
Renin antibody, pepstatin, phospholipids, and substrate analogs, including tetrapeptides and octa- to tridecapeptides, with inhibition constants (Ki) in the 10.sup.-3 to 10.sup.-6 M region, have been studied.
Many efforts have been made to prepare a specific renin inhibitor based on pig renin substrate analogy, which as been shown to correlate well with and predict human renin inhibitor activity. The octapeptide amino acid sequence extending from histidine-6 through tyrosine-13 ##STR1## has been shown to have kinetic parameters essentially the same as those of the full tetradecapeptide renin substrate.
Kokubu e-t al., Biochem. Pharmacol., 22, 3217-3223, 1973, synthesized a number of analogs of the tetrapeptide found between residues 10 to 13, but while inhibition could be shown, inhibitory constants were only of the order of 10.sup.-3 M. Analogs of a larger segment of renin substrate were synthesized,
Burton et al., Biochemistry 14: 3892-3898, 1975, and Poulsen et al., Biochemistry 12: 3877-3882, 1973, but a lack of solubility and weak binding (large inhibitory constant) have proven to be major obstacles to obtaining effective renin inhibitors.
In the case of pepstatin, Umezawa et al , in J. Antibiot. (Tokyo) 23: 259-262, 1970, reported the isolation (from culture filtrates of actinomyces) of that N acylated pentapeptide, (pepstatin), having the structure: ##STR2## This pentapeptide was reported to be an. inhibitor of aspartyl proteases such as pepsin, cathepsin D, and renin, with an I.sub.50 ratio against pepsin and renin generally in the range of 300 to 1000, depending on the sensitivity of the assay. Gross et al., Science 175:656, 1972, reported that pepstatin reduces blood pressure in vivo after the injection of hog renin into nephrectomized rats, but pepstatin has not found very wide application as an experimental agent because of its limited solubility and its inhibition of a variety of other acid proteases in addition to renin.
It has now been found that a novel phenyl derivative of pepstatin is about four times more potent than pepstatin A in inhibiting renin activity and has an I.sub.50 ratio of renin to pepsin which is about ten fold less than that for pepstatin A. This new phenyl derivative, therefore, offers significant therapeutic advantages for the treatment of high blood pressure and congestive heart failure in mammals. This derivative is also a useful starting material for the preparation of statine, a synthetic amino acid, which has been successfully substituted into renin substrates as a peptide bond isostere at the 10-11 position.